1. Field of the Invention
The present invention relates to an image forming apparatus, such a printer, copying machine, facsimile machine, etc., and particularly to a bias application control device for the image forming apparatus, in which a light-image exposing device forms a latent image on the surface of an image carrier uniformly charged by a charger for development by a carrier, wherein a development bias voltage is applied to the carrier to develop the latent image.
2. Description of the Related Art
In an electrophotographic device utilizing a reversal development method, development toner tends to adhere to a photosensitive member serving as an image carrier at the start of the image forming operation.
To prevent toner from adhering to the photosensitive member, starting of operation of the developer is delayed for a period corresponding to the time a charged area on the photosensitive member moves from a position facing the charger to a position facing the developer at the start time of the reversal development operation, as is disclosed in Japanese Laid-Open Publication No. 61-57963/1986.
However, the structure disclosed in Japanese Laid-open Publication No. 61-57963/1986 does not prevent a small amount of toner from adhering to the photosensitive member by fogging. Furthermore, a structure having two development rollers is not disclosed in Japanese Laid-open Publication No. 61-57963/1986.
In the reversal development method, toner tends to adhere to the photosensitive member at the start of the image forming operation and also tends to adhere to the photosensitive member due to fogging under the condition that an electric field for development is not formed. The toner-adhering quantity due to fogging is small per the unit area on the photosensitive member. But the total amount of adhering toner is not small, because toner-adhering continues along the width of the photosensitive member surface, wherein development is possible, during running of the photosensitive member.
Toner adhering on the photosensitive member by fogging is cleaned by a cleaning device, and toner removed by the cleaning device is collected into a toner container. The collected toner in the toner container is mostly due to toner adhering to the photosensitive member due to fogging.
Recently, compact-sizing of the toner container is desirable, because reduction of device size is desired.
Furthermore, in case that the development bias voltage is applied to two development rollers at the same timing at start of the image forming operation, the following problems arise, as described in relation to FIGS. 1a-1d.
As shown in FIG. 1a, upon each operation of a photosensitive member 1 and a charger 2 turn off, application of the development bias voltage does not start to be applied to both the first roller 44 and the second roller 45 on which negative polarity toner 9a and positive polarity carrier 9b are carried.
Next, as shown in FIG. 1b, upon each turn on operation of the photosensitive member 1 and a charger 2, operation of the first roller 44 and second roller 45 starts. The surface of the photosensitive member 1 is uniformly charged at -800 volt by the charger 2 and a leading edge portion of a charged area "A" on the photosensitive member 1 rotates to a position facing the first roller 44. An area "B" is an uncharged area at 0 volt on the photosensitive member 1.
Next, as shown in FIG. 1c, the development bias voltage at -600 volt starts to be applied to both development rollers 44, 45 when the leading edge portion of the charged area on the photosensitive member 1 reaches the position facing the first roller 44 located at the upstream side in the direction of the photosensitive member rotation. Because a weak electric field, which attracts negative polarity toner to the uncharged area "B-1" on the photosensitive member 1, is generated between the photosensitive member 1 and the second roller 45, the toner 9a which is not consumed for development adheres to the uncharged area "B-1" on the photosensitive member 1, wherein the area "B-1" corresponds to a position between the first roller 44 and the second roller 45.
Next, as shown in FIG. 1d, the unnecessary toner adhered area is formed in the "B-1" area.
On the other hand, in case that the development bias voltage is applied to two development rollers at the same timing when the leading edge portion of the charged area on the photosensitive member 1 reaches the position facing the second roller 45 located downstream in the direction of the photosensitive member rotation, the following problems arise, as described in relation to FIGS. 2a-2d.
FIG. 2a is the same as FIG. 1a, and so explanation concerning FIG. 2a is omitted.
As shown in FIG. 2b, the carrier 9b on the first rotating roller 44 tends to adhere to the photosensitive member 1, when the leading edge portion of the negative charged area at -800 volt on the photosensitive member 1 reaches a position facing the first roller 44 located upstream in the direction of the photosensitive member rotation. Because the electric field, which attracts positive polarity carrier 9b to the charged area "A" on the photosensitive member 1, is generated between the photosensitive member 1 and the first roller 44, the carrier-adhering to the photosensitive member 1 continues until starting the application of the development bias voltage to both development rollers 44 and 45. As a result, the carrier 9b adheres to the charged area "A-1" on the photosensitive member 1, wherein the area "A-1" corresponds to the position between the roller 44 and the second roller 45.
As shown in FIG. 2c, the development bias voltage at -600 volt starts to be applied to both development rollers 44 and 45 when the leading edge portion of the area negatively charged at 800 volt on the photosensitive member 1 reaches the position facing the second roller 45 located downstream in the direction of the photosensitive member rotation. The generated electric field between the photosensitive member 1 and the first roller 44 weakens and the carrier-adhering stops.
Next, as shown in FIG. 2d, the carrier-adhered area is formed in the "A-1" area.
On the other hand, at the time of completion of the image forming operation, in case that the development bias voltage ceases being applied to the two development rollers at the same timing, when the trailing edge portion of the charged area on the photosensitive member 1 reaches the position facing the first roller 44 located upstream in the direction of the photosensitive member rotation, the following problems arise, as described in relation to FIGS. 3a-3c.
As shown in FIG. 3a, the development bias voltage at -600 volt continues to be applied to both rotating rollers 44 and 45 and the charger 2 switches from being turned on, in which the surface of the photosensitive member 1 is uniformly charged at -800 volt by the charger 2, to being turned off in which the surface of the photosensitive member 1 is not charged. In this situation, an area "C" is an uncharged area on the photosensitive member 1 and an area "D" is a charged area on the photosensitive member 1.
As shown in FIG. 3b, when the trailing edge portion of the charged area "D" on the photosensitive member 1 reaches the position facing the first roller 44, the development bias voltage ceases to be applied to both development rollers 44 and 45. The electric field, which attracts carrier 9b to the charged area "D-1" on the photosensitive member 1, is generated between the photosensitive member 1 and the first roller 44. In this situation, the charged area "D-1" corresponds to the position between the roller 44 and the second roller 45. As a result, the carrier 9b adheres to the charged area "D-1" on the photosensitive member 1.
As shown in FIG. 3c, when the trailing edge portion of the charged area "D" on the photosensitive member 1 passes the position facing the second roller 45, the carrier-adhering stops.
On the other hand, on completion of the image forming operation, in case that the development bias voltage ceases to be applied to the two rotating rollers 44, 45 at the same timing, when the trailing edge portion of the charged area on the photosensitive member 1 reaches the position facing the second roller 45 located downstream in the direction of the photosensitive member rotation, the following problems arise, as described in relation to FIGS. 4a-4c.
FIG. 4a is the same as FIG. 3a, and so explanation of FIG. 4a is omitted.
As shown in FIG. 4b, after the trailing edge portion of the charged area "D" on the photosensitive member 1 passes the position facing the first roller 44, the electric field, which attracts toner 9a to the uncharged area "C-1" on the photosensitive member 1, is generated between the first roller 44 and the photosensitive member 1, which is at 0 volt. In this situation, the uncharged area "C-1" corresponds to the position between the roller 44 and the second roller 45. As a result, the toner 9a adheres to the uncharged area "C-1" on the photosensitive member 1.
As shown in FIG. 4c, when the trailing edge portion of the charged area "D" on the photosensitive member 1 passes the position facing the second roller 45, the toner-adhering stops.
Japanese Laid-open Publication No. 61-290455/1986 discloses a voltage control circuit for applying a predetermined voltage, which is different from a regular development bias voltage, and continuing to apply the predetermined voltage to the development roller until a predetermined lapse after the rotation of the photosensitive member starts. An operation control circuit stops the operation of the photosensitive member on the lapse of the predetermined period after stopping of charging by the charger.
The inventors have experimentally determined that a considerable quantity of toner adheres to the photosensitive member from the development roller from the moment of the start of operation of the photosensitive member. Application of a voltage of reverse polarity in relation to toner polarity to the development roller at the start of operation of the photosensitive member is not enough to prevent the toner from adhering to the photosensitive member from the development roller.